In synchronous motors (hereinafter simply referred to as “electric motors”) such as permanent magnet motors, synchronous reluctance motors, etc., it is necessary to supply an appropriate current to a stator in accordance with the position of a rotor magnetic pole, and hence a rotor magnetic pole position sensor is fundamentally required for driving such a motor. In the case of using such a rotor magnetic pole position sensor, however, there are problems such as an increase in the cost, reduction in reliability and durability, an increase in electric wiring, etc., so a sensorless control system is desired which uses no rotor magnetic pole position sensor. In order to solve these problems, there has been disclosed, for example, a technique as described in a first patent document (Japanese patent No. 3312472).
A conventional apparatus disclosed in the first patent document includes an alternating voltage impression section that impresses an alternating voltage to an electric motor, a current detection section that detects a motor current, a vector conversion section that divides the detected motor current into a parallel component and a quadrature component with respect to the alternating voltage to be impressed, and a magnetic pole position estimation section that estimates the rotor magnetic pole position of the electric motor based on at least one of the parallel component and the quadrature component of the motor current.
In the above-mentioned conventional apparatus, when there exists a phase difference (phase difference angle θ in the first patent document between the direction in which the alternating voltage is impressed and the direction of magnetic poles, for example, as shown in expression 8 in the first patent document, the position of a rotor magnetic pole is estimated by using a phenomenon that an alternating current with an amplitude proportional to sin 2 θ in a direction (a qc axis direction in the first patent document) orthogonal to the impressed alternating voltage (in a dc axis direction in the first patent document).
The reason for the occurrence of such a phenomenon is that in general, in electric motors with saliency, the inductance in the rotor magnetic pole direction becomes maximum (positive saliency) or minimum (inverse saliency). However, in actual electric motors, even if the direction in which the alternating voltage is impressed and the actual magnetic pole direction coincide with each other, there might be generated an alternating current in a direction orthogonal to the alternating voltage.
FIG. 4 illustrates one such an example with the result of experiments, in which when a rotor magnetic pole position and an alternating voltage impression direction coincide with each other, the change of the amplitude of current in a direction orthogonal to the rotor magnetic pole position according to an alternating voltage is shown.
In FIG. 4, the axis of abscissa represents time [s], and the axis of ordinate represents the rotor magnetic pole position (thin line) in electrical angle [10/360 degrees] and the current amplitude (thick line) [A]. As clear from FIG. 4, it is found that the current amplitude varies in a periodic manner in accordance with the change of the rotor magnetic pole position in spite of that the direction in which the alternating voltage is impressed and the rotor magnetic pole position coincide with each other.
The reason for the generation of this phenomenon is that in an actual electric motor, the direction of the rotor magnetic pole position and the direction of a minimum inductance (or a maximum inductance) do not coincide with each other, and the amount of deviation therebetween varies according to the rotor magnetic pole position.
FIG. 5 shows the section of an embedded permanent magnet electric motor used in the experiment of FIG. 4, and eight rectangular parts of the rotor are permanent magnets embedded therein. The rotor of this electric motor has eight poles, and a stator thereof comprises a concentrated winding armature having twelve slots. It is known that the embedded permanent magnet electric motor is not axisymmetric in the magnetic circuit configuration of the rotor and has electric saliency because of the embedded arrangement of the embedded permanent magnet.
FIG. 6 shows a simplified iron core structure of the electric motor of FIG. 5 while taking out only one pair of poles therefrom. However, note that though the embedded permanent magnet electric-motor of FIG. 5 has inverse saliency in which inductance is minimum in the rotor magnetic pole direction (rotor magnetic pole position), the electric motor model of FIG. 6 has positive saliency in which inductance is maximum in the rotor magnetic pole direction. In this regard, the difference between the inverse saliency and the positive saliency of the electric motor is merely that the magnetic pole directions defined are displaced by 90 degrees in electrical angle from each other.
Here, let us consider the change in inductance of the electric motor according to the axis of observation separately with respect to the stator and the rotor.
First of all, considering the inductance change in case of the absence of saliency in the rotor of the electric motor of FIG. 6, the inductance is uniquely decided by the direction of the observation axis on the stator irrespective of the rotor magnetic pole position, as shown in FIG. 7.
In FIG. 7, an inductance on an observation axis γ=0 and an inductance on an observation axis γ=π/3(=60 degrees) become equal to each other because the relative positional relations between these observation axes and the core of the stator are identical with each other.
In contract to this, the inductance on an observation axis γ=0 and an inductance on an observation axis γ=π/6(=30 degrees) do not necessarily become equal to each other because the relative positional relations between these observation axes and the core of the stator are different from each other.
Here, let us assume that the inductance in the observation axis direction varies on the observation axis γ=0 and on the observation axis γ=π/6(=30 degrees), and that the inductance in the observation axis direction changes monotonously from γ=0 to γ=π/6(=30 degrees) and from γ=π/6(=30 degrees) to γ=π/3(=60 degrees). At this time, it is considered that the change in the inductance according to the observation axis in the model shown in FIG. 7 varies at a period of 60 degrees in electrical angle, as shown by a broken line in FIG. 8.
Although various contrivances are made in the motor design so as to reduce such variation as much as possible, it is particularly difficult to decrease this variation to zero in a concentrated winding armature as shown in FIG. 6.
It is considered that the stator of an electric motor with a large inductance change as stated above has saliency, and, the electric motor of the structure as shown in FIG. 6, of which both the rotor and the stator have saliency, is called a double-salient electric motor.
Next, when considering the change in inductance in case of the absence of saliency in a stator, as shown in FIG. 9, the inductance is decided by an angle between a rotor magnetic pole position and an observation axis.
In FIG. 9, an inductance on an observation axis δ=0 and an inductance on an observation axis δ=π(=180 degrees) become equal to each other because the relative positional relations between these observation axes and the core of the stator are identical with each other.
In contract to this, the inductance on an observation axis δ=0 and an inductance on an observation axis δ=π/2(=90 degrees) do not necessarily become equal to each other because the relative positional relations between these observation axes and the core of the stator are different from each other.
Here, it is considered that assuming that the inductance in the observation axis direction changes monotonously from δ=0 to δ=π/2(=90 degrees) and from δ=π/2(=90 degrees) to δ=π(=180 degrees), the change in the inductance according to the observation axis in the model shown in FIG. 9 varies at a period of 180 degrees in electrical angle, as shown by a thin line in FIG. 8.
The characteristic of the inductance change according to the observation axis of the electric motor shown in FIG. 6 is obtained by combining the above-mentioned two inductance characteristics with each other.
FIG. 8 shows a stator-induced inductance (thin line), a rotor-induced inductance (broken line) and a combined inductance (thick line) when the rotor magnetic pole position θ is zero (θ=0) (a U phase winding and the rotor magnetic pole are confronted with each other). However, in FIG. 8, it is assumed that the magnitude of each inductance is normalized.
In the state of FIG. 8, the direction of the rotor salient pole and the maximum direction of the combined inductance coincide with each other, and a maximum inductance is reached at electrical angles of 0 and π(=180 degrees).
In contrast to this, FIG. 10 shows the characteristic of the inductance change according to the observation axis of each inductance in case of the rotor magnetic pole direction (rotor magnetic pole position) θ=π/12 (=15 degrees).
It is found that in the state of FIG. 10, there arises an amount of deviation (deviation angle) between the rotor magnetic pole direction (the maximum direction of the rotor inductance) and the maximum direction of the combined inductance.
FIG. 11 shows the change of a deviation angle between the rotor magnetic pole direction and the maximum direction of the combined inductance when the rotor magnetic pole direction θ is changed from 0 up to π(=180 degrees).
As is clear from FIG. 11, it is found that the deviation angle periodically changes in a period of an electrical angle of 60 degrees. Accordingly, it is considered that in an electric motor with a double salient pole characteristic as shown in FIG. 6, the deviation angle between the rotor magnetic pole direction and the maximum direction of the combined inductance is generated 6 times or periods during one electrical angle revolution (0–360 degrees).
In addition, as can be seen from FIG. 10, a similar phenomenon occurs with respect to a direction advanced 90 degrees from the rotor magnetic pole direction (i.e., the rotor inverse-salient pole direction) and the minimum combined inductance direction.
Thus, it is considered that, as in the experiments of FIG. 4, when the rotor is driven to run with an alternating voltage being impressed in the rotor magnetic pole direction (i.e., the rotor inverse-salient pole direction), a deviation angle equal to 6 periods is generated for one revolution in electrical angle between the direction of impression of the alternating voltage and the minimum direction of the combined inductance, as a result of which an alternating current is generated in a direction orthogonal to the alternating voltage, as shown in FIG. 4.
Accordingly, the present invention is intended to obviate the problems as referred to above, and provide a magnetic pole position estimation apparatus for a synchronous motor which is capable of estimating the magnetic pole position of a rotor in a precise manner even in an electric motor with so-called double saliency by removing an influence resulting from the double saliency of the electric motor, i.e., an influence of a deviation of the axis of an alternating current due to the rotor magnetic pole position on the estimation of the magnetic pole position.